clarify text alternatives vs. media alternatives for video-only content

understanding/20/audio-only-and-video-only-prerecorded.html

@@ -39,6 +39,8 @@ <h2>Intent of Audio-only and Video-only (Prerecorded)</h2>
          language and learning disabilities to understand the content because it would provide
          parallel presentation.
       </p>
+
+      <p>For video-only content, the 'text alternative' (such as the alt text or aria-label) serves to identify the content, whereas the 'alternative for time-based media' provides the full equivalent information. While these can be the same for very short or simple videos (e.g., a scrolling ticker), they are not required to be identical. For longer or more complex videos, it often improves usability to use the text alternative as a brief label and provide the detailed content description as a separate alternative for time-based media.</p>
 
       <div class="note">
 
@@ -107,6 +109,8 @@ <h2>Examples of Audio-only and Video-only (Prerecorded)</h2>
                the tutorial text for more information.</dd>
          <dt>A video-only file with an audio track</dt>
          <dd>A silent movie includes an audio track which includes a description of the action in the video.</dd>
+         <dt>A complex silent animation</dt>
+         <dd>A web page features a silent animation demonstrating the internal mechanics of a car engine. The text alternative for the video element is "Animation of 4-stroke engine cycle," which briefly identifies the content. A separate link below the video points to a full "alternative for time-based media" (transcript) that describes the intake, compression, combustion, and exhaust strokes in detail.</dd>
       </dl>
 
    </section>